Sensors that make use of ceramic semiconductor materials to detect gases present in air have been known for some time. Whereas hitherto only sensors utilizing reactions involving n-type ceramic semiconductors were known, the inventors associated with the present application have discovered that gases can be detected using high-purity CuO, which is a p-type semiconductor. They have previously filed a patent application for this, which application will hereinafter be referred to as "the prior application." This prior application was laid open to public inspection as Japanese Kokai Patent 6-258270. In the gas sensor disclosed in this prior application, at least 99 wt % of the semiconductor component contributing to conductivity in the sintered product was CuO, and the amount of additive was no more than 1 wt %. It was also disclosed in the prior application that CO sensitivity is increased by adding an alkaline metal compound as this additive which is present at no more than 1 wt %.
However, although CO sensitivity was increased by adding an alkali metal compound to the high-purity CuO, the CO sensitivity thus obtained was only about twice the sensitivity to the same concentration of H.sub.2 for example. A further consideration is that recent research on the connection between global warming and CO.sub.2 gas has highlighted the need for a gas sensor capable of measuring CO.sub.2 gas concentrations.
Meanwhile, detection of gases in exhaust gases differs from detection in air in that the partial pressure of oxygen varies, and CO.sub.2 concentration and water vapor partial pressure vary according to the state of combustion. Under such conditions there are no sensors with high enough sensitivity to CO alone that they can be used for CO detection without some modification. For example, a gas sensor comprising CuO with addition of Na.sub.2 CO.sub.3 is sensitive to CO.sub.2 as well, and at the CO.sub.2 concentrations present in exhaust gas from gas-fired water heaters, its sensitivity to CO.sub.2 is close to its sensitivity to CO at the CO concentrations which have to be detected to give warning of a dangerous amount of CO. This means that selective detection of CO will sometimes be unsuccessful.
For example, in experiments performed by the present inventors, it was found that at a CO.sub.2 concentration of 5.5% an experimental gas sensor comprising CuO with addition of Na.sub.2 CO.sub.3 gave about the same detection output for CO.sub.2 as the output in response to the approximately 2000-4000 ppm of CO which was generated during incomplete combustion. Given that both CO.sub.2 and CO are present in exhaust gas during incomplete combustion, and that even during normal combustion CO.sub.2 is generated at concentrations sufficiently large to be expressed in percent rather than ppm, it will be seen that such high sensitivity to CO.sub.2 is unsuitable for selective detection of CO.
It is an object of the present invention to solve such problems by providing gas sensors capable of selective detection of CO and CO.sub.2 ; gas sensors in which the CO.sub.2 sensitivity has been made lower than the CO sensitivity; and manufacturing methods for these sensors.